The present invention relates to automatic machines for grinding and bevelling lenses of eyeglasses.
The conventional machines for grinding and bevelling ophthalmic lenses comprise a U-shaped carriage mounted to be slidable and oscillable on a fixed horizontal shaft, a second horizontal shaft mounted to be rotatable perpendicularly between the branches of the U-shaped carriage and comprising two parts between which the lens to be machined is clamped, and a third horizontal rotatable shaft carrying the various grinding wheels for trimming and bevelling the edge surface of the lens blank.
When a blank of an optical lens having a thick edge has been shaped in the shape of the eyeglass frame, the edge surface of the lens has a cylindrical shape and a bevelling operation must be carried out by guiding the lens so as to form on its edge surface a V-section bevel between the edges of its periphery, this bevel being intended to be engaged in the groove of the bezel of the frame.
Effecting such a bevel with a manual control is extremely difficult. Use was made of the "free bevel" system in which the edge surface of the blank is fed into a V-section groove of a grinding wheel and the shaft carrying the lens is left free to move in translation during the grinding operation.
However, this system does not give a fully satisfactory result.
The document FR-A-2 456 304 describes a device for measuring the distance between a given point of the desired contour of an ophthalmic lens and the plane tangent to the pole of the convex surface of this lens. This device comprises an assembly having two followers or tracers one of which cooperates with the lens whereas the other cooperates with the template mounted on the carriage and movable in a direction parallel to the axis of the template and of the lens, one of the followers being in contact with the surface of the lens. As the lens rotates, the follower in contact with the surface of the lens delivers an indication by shifting a pointer in front of a graduated index.
In this device, the fact of contacting only a single point of the lens for a given angle of the latter requires effecting a correction calculation, owing to the fact that the grinding point is displaced differently in accordance with the diameter of the grinding wheel and the non-circular shapes of the frames.
The present invention relates to an automatic machine for grinding and bevelling ophthalmic lenses, of the type comprising a non-mechanical type sensor and in which this correction calculation is unnecessary.
A machine of this type is described and illustrated in the document EP-A-0 281 480 B1 in which a follower or tracer is in contact with the edge surface of the lens and its surface is formed by the adjacent edges of a plurality of movable or vibrating elements adapted to act on sensors responsive to their displacements. However design is not fully satisfactory in that, on one hand, its precision is sometimes insufficient and, on the other hand, there are risks of soiling of the articulation means of the movable elements.